joinerLearnsHostRoomIdOnAdmission

A joiner has no identity until it is admitted, then reads the host's — one transition, to the value the host already held.

Sampled before the wait, not after, so this cannot be satisfied by a fabric that admits so fast the null phase is never observable; and the post-condition is keyed to the roster rather than to a roomId collect, so a fabric that never delivers the id fails rather than hanging on a flow that never emits.